Dental defects are defined as varying degrees of damage or abnormalities in the texture and physiological anatomical appearance of the hard tissues of the tooth. When the hard tissue of the tooth is partially missing and large, it is called a crown remnant; when the hard tissue of the tooth is completely missing or nearly completely missing, it is called a root remnant. In the past, a common misconception about the treatment of bad teeth (root and crown remnants) is to pull out the teeth that cannot be filled, and to take the approach of “extraction followed by veneering”, especially for the middle-aged and elderly people who have fewer teeth left in their mouths, most of them think that full denture restoration after all the teeth are pulled out is not only once and for all, but there will be no more teeth that need to be repaired. Moreover, the medical cost of making a full denture is low, which is economical and affordable. In fact, permanent teeth are the second and last set of teeth of human beings, and will not be regenerated after extraction, so we should try to save our teeth, just like Xiao Shen Yang said, this life is like sleeping, eyes closed and opened a day has passed; eyes closed and not opened this life has passed. Teeth in the oral cavity along with our life, plays a very important role, is an indispensable part of the body, so we should effectively preserve it, cherish the teeth to improve the quality of life. 1, the retention of the residual root and crown can slow down the resorption of the alveolar bone and maintain the height of the alveolar bone: although the mechanical stimulation received by the residual crown and root in the chewing movement may be weaker, it can still delay the resorption of the alveolar bone of the affected tooth. If the crown or root is removed, the physiological stimulation of chewing and other functions disappears, and the alveolar bone in the area will then undergo disuse atrophy. If the alveolar bone is less resorbed, it will be beneficial for future denture restoration; if the alveolar bone is too much resorbed and even becomes low, it will cause great difficulties for future denture restoration. 2, the retention of the residual root and crown preserves the periodontal membrane and maintains the physiological function of the tooth: the residual crown and root are extensive defects in the hard tissue of the tooth, but the periodontal membrane tissue still exists and can play a normal physiological role and can withstand a certain amount of chewing pressure. The periodontium is the main connective tissue linking the tooth and the alveolar bone, which not only cushions the pressure received by the tooth, but also transforms it into a traction force that is evenly distributed to the alveolar bone surface, etc. Most of the nerves in the periodontal tissues are proprioceptive nerves, whose main function is tactile and deep pressure sensation. Tactile receptors are very sensitive and can perceive various physical properties of food when the affected tooth is chewing food. The pressure receptors in the periodontium transform the received mechanical signals into electrical signals to the central nervous system, which play a reflexive role in regulating the occlusal force. The retention of root remnants and crowns by perfect root canal treatment can preserve the periodontal membrane proprioceptive receptors, play a supportive role and slow down the rate of alveolar bone resorption. 3. Fixed restoration with root remnants and crowns: beautiful, comfortable, functional, and does not damage adjacent teeth. If the residual root and crown are removed, the alveolar socket will be slow to recover after extraction, and it will be very painful to wait for a long time to set the tooth and eat without teeth for a longer period of time. Removable prosthesis and abutment teeth are often full of food residue and cause decay; fixed bridge prosthesis requires more grinding and modification of adjacent teeth; and total prosthesis is not as good as removable prosthesis and fixed prosthesis with abutment teeth in terms of chewing function or retention. As far as the use of denture is concerned, fixed denture is more comfortable and convenient than removable denture and has better chewing function; removable denture has better retention than total denture. In the case of complete loss of teeth, only total dentures can be chosen. If the alveolar ridge is severely resorbed, especially the retention of the mandibular denture is poor, and the chewing function is very poor, if two or more residual crowns and roots can be properly preserved and restored, the retention capacity can be greatly improved and the chewing function can be improved. 4, the retention of the residual crown and root helps the patient’s psychological health: some patients’ residual crown and root are not effectively preserved, forming a state of missing teeth, which not only affects the main physiological functions of the mouth, but also the alveolar bone is easy to resorb, the face looks old, inaccurate pronunciation and air leakage, easily causing psychological disorders, reluctance to participate in social activities, forming psychological autism, which directly or indirectly affects their life This will directly or indirectly affect their quality of life and work efficiency. 5, elderly treatment: for some elderly people who cannot tolerate tooth extraction (such as suffering from hypertension, diabetes, heart disease and other systemic diseases), coupled with the fear in their hearts, most people are reluctant to remove the residual roots and crowns in their mouths, the harmful roots can be turned into harmless roots through root canal treatment, so that the residual roots can be preserved. 6, the retention and restoration of residual roots and crowns has an important role in maintaining the stability of the dental row: the more the residual roots and crowns are retained, the better the integrity of the dental row can be maintained. 7, the preservation of some important position of the residual root crown, in the anterior teeth can play a role in restoring the aesthetic: to avoid the activity of the restoration affect pronunciation; posterior teeth of the residual root crown, such as the first and second molar residual root crown has important restorative significance, can avoid the free end of the missing active denture repair. As the treatment of crown and root preservation itself is risky, time-consuming, energy-intensive and costly, in this situation, whether to use the treatment of crown and root preservation, in addition to the objective diagnosis of the doctor, the subjective needs of the patient will play an important role. Therefore, before treatment, we should first fully communicate with the patient, and after the patient is informed, agrees, fully understands the treatment procedure, possible problems and how to actively cooperate, further develop the treatment and restoration plan for preserving residual crowns and roots, and then implement it according to the plan. In addition, modern dentistry emphasizes the preservation and restoration of dental pulp organs and residual crowns and roots, not all residual crowns and roots can be preserved. There are strict indications as to whether a residual crown or root can be preserved for restorative treatment. Determining the preservation or extraction of a residual root should be based on the extent of defective damage to the root, the health of the periapical tissue, and a comprehensive consideration of the relationship between the treatment outcome and the restoration. If the destruction of the residual root is large, the defect reaches the subgingival level, the lesion of the periapical tissue is more extensive, and the treatment effect is poor, extraction can be considered; if the residual root is more stable, the periapical tissue has no obvious lesion or the lesion is less extensive, and it is also useful for the support and fixation of the denture, it should be retained after root canal treatment. Retention and restoration of residual crowns and roots requires a comprehensive series of treatments by a combination of dental, endodontic, periodontal and alveolar surgery, orthodontics, prosthodontics and other related specialties to form a comprehensive treatment consortium. The following patients with systemic diseases may have some problems in restoring missing teeth and using the denture after extraction, and the residual crowns and roots of these patients should not be extracted easily: (1) the problem of denture support and retention for diabetic patients; (2) the problem of movable denture retention for patients with dry mouth; (3) the problem of denture safety for patients with mental and neurological diseases; (4) the problem of denture for patients with osteoporosis; ( (5) prosthesis for patients with hand dysfunction due to systemic disorders or trauma, etc. At present, the restoration methods of the residual root and crown include pile crown, overdenture, inlay, etc. The residual root and crown can be treated with perfect root canal treatment, thorough periodontal treatment and combined periodontal and endodontic treatment. For the longer residual roots in the bone, they can be used for piling and crowning. For shorter roots, the crown can be restored together with the adjacent teeth to increase its support and retention. This kind of denture is functionally closer to the real teeth, less foreign body sensation after restoration, high chewing efficiency, more comfortable to use, and can feel the size and taste of food when chewing, and control the force and direction of biting, so that patients quickly adapt to the use of dentures. Compared with the fixed denture or implant prosthesis after root extraction, it has the advantages of short treatment time, no trauma and low cost. For those whose roots are too short for pile crown restoration, root canal treatment can be performed followed by overdenture restoration. (There are some patients who cannot be extracted due to serious systemic diseases, such as hypertension, coronary heart disease, diabetes, etc., and whose roots are too calcified for root canal treatment, they can also be restored with overdenture. If the tooth root appears to be compressed, the tissue surface of the denture corresponding to the root can be cushioned to make light contact.) Case analysis Case 1, patient, female, 45 y. Complaint: left lower posterior tooth with residual crown, request to preserve the affected tooth. Examination: The patient had a left lower posterior crown, one week after root canal treatment, and now there is no percussion pain, spontaneous pain, and no sinus tract. Only the proximal mesial and proximal buccolingual lateral walls remain. Restoration method: single root canal pile crown restoration Case 2, patient, male, 68 y. Complaint: left upper posterior tooth with residual root, request to preserve the affected tooth. Examination: The patient had a left upper posterior tooth with a distal mesial residual root section about 0.5 mm below the gingiva and a small proximal mesial supragingival tooth remaining. Restoration method: double root canal pile core crown restoration In summary, in general, do not remove residual crowns and roots easily. With the development of material science, treatment, and prosthetics, reasonable retention of the residual root and crown can greatly improve the chewing efficiency after denture restoration and improve the quality of life of patients. The value of preserving residual roots and crowns is even more important, so all residual roots and crowns with retention value should be preserved by all means to improve the prosthetic effect of the denture.